In many applications, high reliability of power converters is required. In voltage source converters, the capacitor bank, in the following referred to as cell capacitor, is one of the major parts causing failures of the power converter because the often implemented electrolytic capacitors have only a limited lifetime.
Especially in modular multilevel converters (MMC), aging of the cell capacitors is a particular problem as the capacitors are subject to high currents at multiples of the fundamental and at the switching frequency, leading to early aging and capacitance drop. As such modular multilevel converters are designed for high reliability, health monitoring of the cell capacitor is of major importance.
During the aging process of electrolytic capacitors, the capacitance drops continuously and the equivalent series resistance (ESR) increases. Many manufacturers define the end-of-life of their capacitors if the initial capacitance has dropped by 20% and/or the ESR has increased by 200-300%.
US2010/0321038 discloses a device for monitoring the aging of the capacitors in a power converter comprising a number of converter cells, by means of capacitance measurement. For each phase module a branch current sensor for detecting a phase module branch current is provided. Furthermore, for each submodule there exists a submodule sensor for detecting a capacitor voltage dropped across the capacitor, means for detecting the switching states of the submodule, and an evaluation unit for determining the capacitance of the capacitors of each phase module. The determining of the capacitance is dependent on the integration of the measured phase module branch current, the switching state or states and the capacitor voltage. This yields the charge change of the capacitors. The capacitance of each capacitor results from the division of the charge change with the voltage change. The charge and voltage are detected between the turn-on and turn-off instants of the submodule.
The above-described method relies on both current and voltage measurements, and especially on computations such as integration of the detected branch currents in order to determine the capacitance.